The laminating of the material and its subsequent curing in an autoclave in a mold providing the geometry of the target piece is required for the curing of a composite piece. Bonds occur between the resin molecules during the curing cycle, obtaining the desired mechanical characteristics of the material. This requires subjecting the material to a temperature pressure cycle inside an autoclave, confining the sheet in an enclosure on a pressure-tight tooling and closed by a vacuum bag.
The geometry of the tooling at the temperature at which the resin hardens must be the final geometry of the piece considering the thermal expansion coefficients of the material itself and of the tooling. Correction coefficients of the geometry of the tooling are applied to account for this effect, an added difficulty to the design thereof. The ideal situation is to use a material for manufacturing the tooling that has the same expansion coefficient as that of the piece once it is cured given that, in this manner, from the moment in which the resin hardens, and during the cooling, the geometry of the piece will be perfectly adapted to the tooling and the references traced on the tooling will be marked on the piece in its correct position. This must be compatible with the fact that the material used to manufacture the toolings must have the smallest thermal mass possible, preserving certain mechanical and durability characteristics which allow its use in an industrial process, particularly including subjecting it to autoclave cycles with no loss of said characteristics.
The use of thermosetting metallic materials such as, among others, those commercially called INVAR 36, NILO 36, allows achieving the characteristics necessary for the manufacture of toolings for autoclave curing, although it has a series of drawbacks with respect to other conventional metallic materials also used in tooling, such as construction steels. The following must be pointed out among the most important comparative disadvantages affecting the process: fewer mechanical characteristics, less diffusivity, greater thermal mass and a much higher raw material cost.